Minor blood group antigens (e.g., Jk, Rh, Kell, Kidd and Duffy) can evoke autoantibodies and/or alloantibodies when administered to a sensitized host, causing auto immune hemolytic anemia, hemolytic anemia, hemolytic disease of the newborn and hemolytic transfusion reactions. Sensitization can result from previous transfusions or pregnancy with an antigen-positive fetus. These alloantibodies are ordinarily detected and identified by testing recipient sera, sequentially in separate agglutination assays, against a panel of red blood cells (RBC) of known phenotypes (for example anti-Jka antibody can be identified by determining if patient serum will react with Jka+ RBC and not with Jka− RBC). By selecting multiple samples of RBC of various antigen phenotypes, it is possible to detect the presence or absence of clinically important alloantibodies.
Agglutination assays are conventionally performed in solution, i.e. in a test tube. Interpretation of data for tube agglutination reaction requires skilled and experienced technologists, especially when the reactions are weaker. However, in recent years, newer technologies such as the gel format and coated solid phase format have been developed. The gel technology is based on the principle of controlled agglutination of red cells through a dextran-acrylamide gel [Judd W J, Steiner E A, Knafl P C, Masters C. The gel test: use in the identification of unexpected antibodies to blood group antigens. Immunohematology 1998; 14:59-62]. Microtubes are filled with a mixture of gel, buffer and reagent. At the top of the gel, a mixture of patient serum and various known red cells are added, followed by centrifugation through the gel under controlled conditions. In a negative reaction, the cells pass through the gel and pellet in the bottom of the microtube. Conversely in positive reactions, the red cells are trapped at various levels in the gel, with the strongest reaction (largest agglutinated mass) giving minimal to no observable migration, with most red cells trapped at or near the top of the gel microcolumn. The solid phase system developed by Immucor provides microwells for immobilization of human erythrocytes, and is used in solid phase assays for detection of IgG red cell antibodies to corresponding red cell antigens [Plapp F V, Sinor L T, Rachel J M, et al A solid phase antibody screen. Am J Clin Pathol 1984; 82:179]. Wells are coated with a chemical coupling agent that allows the immobilization of user selected red cells to the microwell surface. Coated wells are incubated with blood products, including sera, plasma or other reagents, under conditions that facilitate antigen-antibody reaction. Following incubation, unbound residual immunoglobins are rinsed from the wells and anti-IgG coated indicator red cells added. Centrifugation brings indicator red cells in contact with antibody bound to the immobilized red cell layer. In case of a positive test, IgG-anti-IgG complexes form between the indicator red cells and the sensitized, immobilized cells. As a consequence of antibody bridging, the indicator cells adhere to the immobilized cells as a second immobilized layer. In the absence of detectable antigen-antibody interactions (negative test), the indicator red cells do not bind to the immobilized cells and pellet to the bottom of the wells as tightly-packed buttons. The plasma membrane forms the interface between eukaryotic cell interiors and the external environment. Consequently, the functions of proteins embedded in this membrane are varied and include cell-cell and cell-extracellular matrix recognition, reception and transduction of extracellular signals, and the transport of solutes and water molecules into and out of the cell. The heterogeneity of the cell surface protein population often translates into difficulties in development of in-vitro assays using extracted membrane proteins or crude cell lysates. The erythrocyte membrane is a complicated structure consisting of a membrane bilayer, an array of embedded proteins and glycoproteins and a complex layer of cytoskeletal protein network, which is known to be sensitive to external conditions [Steck, T. L. The organization of proteins in the human red blood cell membrane. J. Cell Biol. vol. 62 (1974) 1-19; Byers, T. L. and Branton, D. Visualization of the protein associations in the erythrocyte membrane skeleton. Proc. Natl. Acad. Sci. USA vol.82 (1985) 6153-6157; Seeman, P., Cheng, D., and Iles, G. H. Structure of membrane holes in osmotic and saponin hemolysis J. Cell. Biol. vol. 56 (1973) 519-527].
Using encoded intact cells as a probe offers an attractive alternative way to screen for ligands that bind to cell surface receptors, and affords development of miniaturized and multiplexed platforms amenable to high-throughput techniques.
Encapsulation of solutes within red blood cells has been widely studied as a means of drug delivery and targeting [Ihler G. M., Glew, R. H. and Schnure, F. W. Enzyme loading of erythrocytes. PNAS vol. 70 (1973) 2663-2666; DeLoach, J. R., Harris, R. L. and Ihler, G. M. An erythrocyte encapsulator dialyzer used in preparing large quantities of erythrocyte ghosts and encapsulation of pesticide in erythrocyte ghosts. Analytical Biochemistry vol.102 (1980) 220-227; Baker, R. F. Entry of ferritin into human red cells during hypotonic haemolysis. Nature vol.215 (1967) 424-425; Marsden, N. V. B., and Ostling, S. G. Accumulation of dextran in human red cells after haemolysis. Nature vol. 184 (1959) 723-724]. In this approach, advantage is taken of the fact that mild osmotic hemolysis induces changes in the membrane porosity of the red cells which allows probes of approximately the same size as proteins and small solutes to partition into the interior of the red cells. By appropriately manipulating the ionic strength (restoration of isotonic conditions) after hemolysis, the pores can be resealed, trapping the partitioned solute permanently in the red cell (also known as erythrocyte ghosts). Washing the resealed ghosts removes excess solute from the external medium. Loading of fluorescently labeled dextran to give fluorescent erythrocyte ghosts using this approach has also been reported in literature [Doberstein, S. K. et al. Fluorescent erythrocyte ghosts as standards for quantitative flow cytometry. Cytometry vol. 20 (1995) 14-18].
The use of resealed ghosts in an assay, however, requires that the cell surface proteins and their orientation are unaffected by the ghost preparation and the solute loading process. Specifically the use of low ionic strength buffer or absence of magnesium ions in the buffer may lead to disaggregation of the molecular components and inside-out folding of the membrane, rendering the product unusable for the current purpose.
Alternate methods of fluorescently encoding cells include use of membrane permeable lipophilic dyes [Tanaka, Y. and Schroit, A. J. Insertion of fluorescent phosphatidylserine in the plasma membrane of red blood cells. J. Biol. Chem. vol. 258 (1983) 11335-11343; Tokumasu, F. and Dvorak, J. Development and application of quantum dots for immunochemistry of human erythrocytes. Journal of Microscopy, vol. 211 (2003) 256-261] and irreversible covalent linking of reactive dyes to the cell surface [Donald, M. M. et al. RBC's labeled at two biotin densities permit simultaneous and repeated measurements of circulating RBC volume. Transfusion, vol 44 (2004) 431-437; Suzuki, T. and Dale, G. L. Biotinylated erythrocytes: In-vivo survival and in vitro recovery. Blood, vol. 70 (1987) 791-795]. Little if any data exists about the effect of covalent attachment on ligand-receptor interactions [Cowley, H., et al. Biotinylation modifies red cell antigens. Transfusion, vol. 39 (1999) 163-168.]. In addition, large numbers of distinct codes are difficult to construct using the surface encoding approach, unless, where a few dyes are used to generate a number of different colors, the encoding reaction is closely regulated, or, a large library of dyes with different spectral fingerprints is used. Thus, an encoding method in which only few dye colors can be used, without close monitoring of the reaction is desired.
In a multiplexed assay format using encoded ghost cells and a secondary antibody to indicate binding of antibodies in the sample to the ghosts, decoding of an array of ghost cells can be done, e.g., with flow cytometry [Wagner, F. F. and Flegel, W. A. Principles and applications of red blood cell flow cytometry. Transfusion Medicine and Hemotherapy vol.25 (1998); Roback, J. D., Barclay, S. and Hillyer, C. D. An automatable platform for accurate Immunohematology testing by flow cytometry. Transfusion Vol. 43 (2003) 918; Roback, J. D., Barclay, S. and Hillyer, C. D. Improved method for fluoresce cytometric immunohematology testing. Transfusion vol. 44 (2004) 187; Sharon, R. and Fibach, E. Quantitative flow cytometric Analysis of ABO Red Cell Antigens. Cytometry vol. 12 (1991) 545-549; Arndt, P. A. and Garratty, G. Flow cytofluorometric analysis in red blood cell immunology. Transfusion Medicine and Hemotherapy vol.31 (2004)]. Other decoding methods, which allow for in-situ decoding with its concomitant advantages of higher throughput, are desirable.